Halep beats Suarez Navarro

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Juan Martin del Potro of Argentina returns a shot to Dominic Thiem of Austria during his fourth round Men's Singles match on Day Eight of the 2016 US Open at the USTA Billie Jean King National Tennis Center on Sept. 5, 2016, in the Flushing neighborhood of the Queens borough of New York City.

Juan Martin del Potro is the lowest-ranked man to reach the U.S. Open quarterfinals since Jimmy Connors in 1991, and it turns out a 27-year-old Argentine can share much in common with a 39-year-old American.

At its root, the story captivating fans at Flushing Meadows is the same: the former champion making an unlikely run to reclaim past glory.

In del Potro's case, his body has betrayed him through injuries, not age — multiple left wrist surgeries that kept him out of the last two U.S. Opens. The only thing missing from his tale so far this year is a marathon match.

Monday's victory was particularly brief after eighth-seeded Dominic Thiem retired in the second set because of a bum knee. For once, an injury was aiding del Potro, not hindering him.

Still tired after the stunning run to an Olympic silver medal that announced him as a contender again, he got a huge break Monday in spending only 72 minutes on court. Del Potro has played just 10-plus sets through four rounds.

Thiem took a medical timeout to have trainers look at his right knee after the second set's fifth game. He returned to the court for one point before deciding he couldn't go on.

Del Potro had won the first set 6-3 and was up a break in the second. He had trainers massage his right shoulder in the first set but looked fine after coming back from down an early break.

Thiem said his knee started hurting three days earlier, and he speculated it was the indirect result of blisters on his feet — he needed to alter his gait to compensate. Late in the first set Monday, it started getting worse. He didn't want to take any risks with a knee injury and planned to get an MRI.

"I couldn't bend it too much the last three days," he said. "So I was all the time a little bit handicapped."

Thiem, who turned 23 on Saturday, has had a breakthrough year, reaching the French Open semifinals and cracking the top 10. But all those victories came with a cost — a tour-high 69 matches, and the long season had started to wear on his body. On top of that, he had played 12 sets through three rounds at the U.S. Open.

"I never expected to play that many matches, of course, this year," he said. "For sure next year I'm going to change it up a little bit."

Del Potro could face third-seeded Stan Wawrinka, who he defeated at Wimbledon, in the quarterfinals. On the 25th anniversary of the 174th-ranked Connors' rousing run to the semis, he's a win away from that stage at No. 142 in the world.

Del Potro came onto Arthur Ashe Stadium on Monday and pointed to an Argentine flag in the stands. But the roars he keeps drawing pour down from just about everyone.

Unlike the Olympics, when del Potro expressed doubt he could make a deep run even after upsetting No. 1 Novak Djokovic, he keeps talking about sticking around at Flushing Meadows. Del Potro recalled in his on-court interview after the match how sad he was to watch the U.S. Open on TV, stuck at home the last two years.

This time, he doesn't want the cheers to end.

"I really don't know what's going on," del Potro said. "We have a good connection between the New York City people, between the Argentine fans the American fans and me."

In other U.S. Open news, Simona Halep made the quarterfinals for the second straight year, beating Carla Suarez Navarro in straight sets.

The fifth-seeded Romanian won 6-2, 7-5 on Monday. She could face No. 1 Serena Williams next. The 22-time major champ plays Yaroslava Shvedova later in the day.